The demodulation process according to the invention may be used in receivers, preferably in relation to transmissions in which the noise power density spectrum of a transmission channel used, as a function of the frequency f, is not constant, and its characteristic is also not known to the receivers. Under those circumstances it is not possible to select two frequencies of a frequency shift keying process, so that the noise power density spectra of the transmission channel are as small as possible at the two frequencies, that is to say, two shifted signals of the frequency shift keying process, which belong to the two frequencies and which are both a respective function of the time are disturbed as little as possible. On the contrary it is to be expected that--if the two frequencies are close together, as is usual in a conventional frequency shift keying process--both shifted signals occur in a severely disturbed frequency range. For such situations, the use if possible of a spread band process (or "spread spectrum" process) is recommended in the literature, for example in the book "Digital Communications and Spread Spectrum Systems", R. E. Ziemer and R. L. Peterson, Macmillan Publishing Company, New York, 1985, pages 327 to 328. However such processes suffer from the disadvantage that they are expensive to carry into effect.
A process of the kind to which this invention relates is known from the publication IEEE Trans. on Communications, Sept. 86, Vol. COM-34, No. 9 "Minimax Noncoherent Detection of BFSK Signals in Nonwhite Gaussian Noise", T. Schaub and B. Steinle, pages 955 to 958, in which it is shown that if the ratio of the noise power densities at the two frequencies is not unity (see FIG. 1), a conventional noncoherent frequency shift keying receiver supplies the same reception results as if the two shifted signals were disturbed, with the same mean noise power density. Also shown therein is a possible solution as to the way in which, for the case of non unity noise power density ratio, the bit error probability of the transmitted binary data can be reduced, in comparison with the case where the ratio is unity. For that purpose, two weighting factors, with which the two shifted signals are multiplied, after they have first been previously separated from each other in the receiver and amplitude-modulated, are optimised.
The invention is based on the problem of improving the known demodulation process and realising a demodulation process which combines the simplicity of the noncoherent demodulation process with the advantages of the spread spectrum process.